Fuel cell system with regulation of DC/DC converter passing power

ABSTRACT

A fuel cell system capable of improving the voltage controllability of a converter provided in the system is provided. A controller judges whether or not a passing power of a DC/DC converter falls within a reduced response performance area for the number of active phases as of the present moment. When the controller determines that the passing power of the DC/DC converter falls within the reduced response performance area, the controller determines the number of phases which avoids the driving within the reduced response performance area, and outputs a command for switching to the determined number of phases (phase switching command) to the DC/DC converter.

This is a 371 national phase application of PCT/JP2008/073052 filed 18Dec. 2008, which claims priority to Japanese Patent Application No.2007-333027 filed 25 Dec. 2007, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system that controls anoutput voltage of a fuel cell by means of a DC/DC converter thatincreases/decreases an output voltage of a fuel cell stack, and a mobileobject equipped with the system.

BACKGROUND ART

A fuel cell stack is an energy conversion system for converting chemicalenergy to electric energy through an electrochemical reaction caused bysupplying a fuel gas and an oxidant gas to a membrane-electrodeassembly. In particular, a solid polymer electrolyte fuel cell stack inwhich a solid polymer membrane is used as an electrolyte is low in cost,easy to be reduced in size, and high in output density, and therefore isexpected to be used as a vehicle-mounted power source.

Regarding a fuel cell system equipped with such a fuel cell stack, forexample, cited reference 1 below proposes a method of enhancing theoperation efficiency of a system by setting, with the use of a DC/DCconverter, an output of a fuel cell to account for 65% to 80% of theentire output.

Patent Document 1: JP2002-118979 A

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, regarding a DC/DC converter that controls an output voltage ofa fuel cell stack to increase/decrease through a switching operation bya switching device, an operating range where a dead time compensationvalue varies greatly depending on the value of a passing power exists.In the operating range where the dead time compensation value variesgreatly (for convenience, hereinafter referred to as a reduced responseperformance area), the response performance of the DC/DC converter isknown to be reduced. The DC/DC converter driven with the reducedresponse performance area has invited, e.g., a problem in that theenergy efficiency of the entire system degrades due to poorcontrollability of an output voltage of the converter (hereinafterreferred to as voltage controllability of the converter).

The present invention has been made in light of the above circumstances,and an object of the invention is to provide a fuel cell system in whichthe voltage controllability of a converter provided in the system can beimproved.

Means for Solving the Problem

In order to solve the problem described above, provided according to anaspect of the present invention is a fuel cell system comprising: amultiphase DC/DC converter for increasing/decreasing an output voltageof a fuel cell; a setting unit that sets the number of active phases forthe DC/DC converter; a calculation unit that calculates a passing powerof the DC/DC converter; a number-of-phases control unit that, when thecalculated passing power falls within a reduced response performancearea, switches from the number of active phases as of the present momentto the number of active phases in which the passing power does not fallwithin the reduced response performance area; and a drive control unitthat drives the DC/DC converter with the number of phases after theswitching.

With such a configuration, when the calculated passing power of theDC/DC converter falls within the reduced response performance area forthe number of active phases as of the relevant moment, the number ofactive phases for the DC/DC converter 60 is switched to the number ofactive phases in which the passing power does not fall within thereduced response performance area. This enables the drive of the DC/DCconverter which avoids the reduced response performance area, therebyimproving the voltage controllability of the DC/DC converter compared torelated art.

Provided according to an aspect of the present invention is a fuel cellsystem comprising: a DC/DC converter for increasing/decreasing an outputvoltage of a fuel cell; a calculation unit that calculates a passingpower of the DC/DC converter; a power control unit that determines, whenthe calculated passing power falls within a reduced response performancearea, a passing power of the DC/DC converter so that the passing powerdoes not fall within the reduced response performance area; and a drivecontrol unit that drives the DC/DC converter so as to obtain thedetermined passing power.

With such a configuration, when the calculated passing power of theDC/DC converter falls within the reduced response performance area, thepassing power of the DC/DC converter is shifted such that the passingpower does not fall within the reduced response performance area. Thisenables the drive of the DC/DC converter which avoids the reducedresponse performance area, thereby improving the voltage controllabilityof the DC/DC converter compared to related art in a similar way to theabove.

Further, provided according to an aspect of the present invention is afuel cell system comprising: a DC/DC converter for increasing/decreasingan output voltage of a fuel cell; a calculation unit that calculates apassing power of the DC/DC converter; a setting unit that sets a carrierfrequency of a control signal for controlling a switching operation ofthe DC/DC converter; a frequency control unit that changes, when thecalculated passing power falls within a reduced response performancearea, the carrier frequency as of the present moment to a carrierfrequency with which the passing power does not fall within the reducedresponse performance area; and a drive control unit that drives theDC/DC converter with the carrier frequency after the change.

With such a configuration, when the calculated passing power of theDC/DC converter falls within the reduced response performance area, thecarrier frequency of the DC/DC converter as of the present moment ischanged to a carrier frequency with which the passing power does notfall within the reduced response performance area. This enables thedrive of the DC/DC converter which avoids the reduced responseperformance area, thereby improving the voltage controllability of theDC/DC converter compared to related art in a similar way to the above.

Further, provided according to an aspect of the present invention is afuel cell system comprising: a multiphase DC/DC converter forincreasing/decreasing an output voltage of a fuel cell; a detection unitthat detects a request voltage for the fuel cell; a parameter controlunit that controls at least one or more parameters, the number of activephases, a passing power and a carrier frequency of the DC/DC converterbased on the request voltage; and a drive control unit that drives theDC/DC converter in accordance with the controlled parameters.

In the configuration above, it is preferable that the fuel cell systemfurther comprises a judgment unit that judges whether or not the requestvoltage exceeds a preset threshold, wherein, when the request voltageexceeds the preset threshold, the parameter control unit controls atleast one or more parameters, the number of drive phases, the passingpower and the carrier frequency of the DC/DC converter based on therequest voltage.

Further, provided according to an aspect of the present invention is amobile object comprising, as a power source, a fuel cell system providedwith a fuel cell and a multiphase DC/DC converter forincreasing/decreasing an output voltage of the fuel cell, the mobileobject comprising: a sensor that detects an acceleration opening degreeof the mobile object; a parameter control unit that controls at leastone or more parameters, the number of active phases, a passing power anda carrier frequency of the DC/DC converter based on the detectedacceleration opening degree; and a drive control unit that drives theDC/DC converter in accordance with the controlled parameters.

Effect of the Invention

According to the present invention, the voltage controllability of aconverter provided in a system can be improved.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments according to the present invention will hereinafter bedescribed with reference to the drawings.

FIG. 1 shows a primary configuration of a fuel cell system 10 accordingto an embodiment. The fuel cell system 10 is a vehicle-mounted powersupply system provided in a power supply system of a fuel cell vehicle.The fuel cell system 10 includes a fuel cell stack 20, an FC auxiliaryapparatus 21, a cell voltage detector 22, a traction inverter 30, atraction motor 40, a secondary battery 50, a DC/DC converter 60, avehicle auxiliary apparatus 70, a controller 80 and sensors 90.

The fuel cell stack 20 is a power generation apparatus having a stackconfiguration in which a plurality of cells is connected in series, thecells each being constituted by arranging a pair of electrodes (anode,cathode) with a solid polymer electrolyte sandwiched therebetween.Hydrogen ions that are generated at the anode through a catalystreaction pass through the solid polymer electrolyte membrane, move tothe cathode, and cause an electrochemical reaction with an oxidant gasat the cathode, generating electric power.

The FC auxiliary apparatus 21 includes a fuel gas supply system(hydrogen storage tank, hydrogen cutoff valve, hydrogen supply pressureregulator, etc.) for supplying a fuel gas (hydrogen gas) to the anode ofthe fuel cell stack 20, an oxidant gas supply system (air compressor,etc.) for supplying an oxidant gas (air) to the cathode of the fuel cellstack 20 and other auxiliary apparatuses (humidifying module forhumidifying the fuel gas and the oxidant gas, fuel cell coolingapparatus, etc.).

Upon receiving the supply of the fuel gas and oxidant gas from the FCauxiliary apparatus 21, the fuel cell stack 20 outputs electric energythrough an electrochemical reaction.

The traction motor 40 is an electric motor for obtaining a traveldriving force, and is constituted by, e.g., a three-phase synchronousmotor.

The traction inverter 30 includes, e.g., a three-phase bridge circuitconstituted by six power transistors, and converts direct-current powersupplied from the fuel cell stack 20 or the secondary battery 50 toalternating-current power (three-phase alternating current) through aswitching operation of the power transistors and supplies thealternating-current power to the traction motor 40. The controller 80has a function of controlling a power conversion operation of thetraction inverter 30, and outputs, for example, respectivealternating-current voltage command values of a U-phase, V-phase andW-phase as switching commands to the traction inverter 30, and controlsthe output torque and revolution speed of the traction motor 40.

The secondary battery 50 is a power storage device that is capable ofstoring and discharging power, and functions as a storage source ofregenerated energy during braking regeneration and an energy bufferduring a load variation as a result of acceleration or deceleration ofthe fuel cell vehicle. The secondary battery 50 is preferablyconstituted by, for example, a nickel/cadmium battery, a nickel/hydrogenbattery, or a lithium secondary battery.

Instead of the second battery 50, a capacitor (electric double layercapacitor, electrolytic capacitor, etc.) as a power storage device maybe connected to a primary side of the DC/DC converter 60.

The DC/DC converter 60 is a voltage conversion unit for controlling anoutput voltage of the fuel cell stack 20 or the secondary battery 50 toincrease/decrease. The DC/DC converter 60 has a circuit configuration ofa multiphase converter in which a circuit similar to an inverter, whichconverts an input voltage (direct-current voltage) to analternating-current voltage, is combined with a circuit that rectifiesthe alternating current and converts it to an output voltage(direct-current voltage). More specifically, the DC/DC converter 60 hasa circuit configuration of a three-phase full-bridge converterconstituted by twelve IGBT devices Tr1 to Tr12, twelve diode devices D1to D12, three reactors L1 to L3 and two smoothing capacitors C1 to C2.

When the DC/DC converter 60 has a low passing power, a single-phaseoperation is carried out since it involves a low switching loss comparedwith a three-phase operation. When carrying out the single-phaseoperation, the pair of IGBT devices Tr1 and Tr10 and the pair of IGBTdevices Tr4 and Tr7 operate. Meanwhile, when the DC/DC converter 60 hasa high passing power, a three-phase operation is carried out since itinvolves a low switching loss compared with a single-phase operation.When carrying out the three-phase operation, the pair of IGBT devicesTr1 and Tr10 and the pair of IGBT devices Tr4 and Tr7; the pair of IGBTdevices Tr2 and Tr11 and the pair of IGBT devices Tr5 and Tr8; and thepair of IGBT devices Tr3 and Tr12 and the pair of IGBT devices Tr6 andTr9 operate with a phase difference of 120 degrees.

The secondary battery 50 is connected to the primary side of the DC/DCconverter 60, while the fuel cell stack 20, the traction inverter 30 andthe vehicle auxiliary apparatus 70 are connected in parallel to asecondary side of the DC/DC converter 60.

For example, the DC/DC converter 60 controls an operating point (outputvoltage, output current) of the fuel cell stack 20 byincreasing/decreasing the output voltage of the secondary battery 50.The DC/DC converter 60 increases the output voltage of the secondarybattery 50 to supply direct-current power to the traction inverter 30when the fuel cell vehicle performs a powering driving by means of thetraction motor 40, and on the other hand, decreases the regenerateddirect-current voltage and charges the secondary battery 50 with theresultant voltage when the fuel cell vehicle performs regenerativebraking by the traction motor 40. The DC/DC converter 60 also has afunction of decreasing the output voltage of the fuel cell stack 20 andcharging the secondary battery 50 with the resultant voltage in order tostore surplus power of the fuel cell stack 20.

The vehicle auxiliary apparatus 70 includes various auxiliaryapparatuses such as a compressor motor for pressurizing an oxidant gas,a pump drive motor for supplying pure water to a humidifying module, acoolant pump drive motor for cooling the fuel cell stack 20 and aradiator fan motor.

The controller 80 is a control unit including a central processing unit(CPU), storage devices (ROM, RAM), an input/output interface, etc. Thecontroller 80 controls the fuel cell vehicle based on, e.g., varioussignals output from the sensors 90. The sensors 90 include, e.g., anignition switch 91, a vehicle speed sensor 92 and an acceleration sensor93.

For example, when receiving an ignition signal output from the ignitionswitch 91, the controller 80 starts the operation of the fuel cellsystem 10, and obtains the power required for the entire system based onan acceleration-opening-degree signal output from the accelerationsensor 93, a vehicle speed signal output from the vehicle speed sensor92, etc. The power required for the entire system corresponds to thetotal value of vehicle driving power and auxiliary-apparatus power. Theauxiliary-apparatus power includes, e.g., power consumed byvehicle-mounted auxiliary apparatuses (humidifier, air compressor,hydrogen pump, coolant circulation pump, etc.), power consumed bydevices necessary for vehicle driving (change gear, wheel controldevice, steering device, suspension device, etc.), and power consumed bydevices arranged in an occupant space (air-conditioning device,illumination device, audio equipment, etc.).

The controller (calculation unit) 80 determines the distribution ofoutput power (i.e., power distribution) between the fuel cell stack 20and the secondary battery 50, and then controls the FC auxiliaryapparatus 21 so that the amount of power generated by the fuel cellstack 20 corresponds to target power to adjust the supply of reactiongas to the fuel cell stack 20 and also controls the DC/DC converter 60to adjust the output voltage of the fuel cell stack 20, therebycontrolling the operating point (output voltage, output current) of thefuel cell stack 20. Moreover, in order to obtain a target vehicle speedin accordance with an acceleration opening degree, the controller 80outputs, for example, respective alternating-current voltage commandvalues of a U-phase, V-phase and W-phase as switching commands to thetraction inverter 30, and controls the output torque and revolutionspeed of the traction motor 40.

FIG. 2 is a graph showing the relationship between a passing power and adead time compensation value of the DC/DC converter 60 for each of thenumbers of active phases. The dotted line indicates the graph for athree-phase operation (three-phase drive), and the solid line indicatesthe graph for a single-phase operation (single-phase drive). In thethree-phase drive, the dead time compensation value varies greatly inthe operating range of around −5 kW (e.g., −5 kW±α1 kW) and theoperating range of around 5 kW (e.g., 5 kW±α1 kW), and thus these twooperating ranges each correspond to a reduced response performance area.Meanwhile, in the single-phase drive, the dead time compensation valuevaries greatly in the operating range of around −2.5 kW (e.g., −2.5kW±α2 (<α1) kW) and the operating range of around 2.5 kW (e.g., 2.5kW±α2 kW), and thus these two operating ranges each correspond to areduced response performance area. As described above, the reducedresponse performance areas differ depending on the number of activephases for the DC/DC converter 60. Therefore, even with the same passingpower, a drive control that avoids the reduced response performanceareas (hereinafter referred to as reduced-performance area avoidancecontrol) can be attained by changing the number of active phases.

Note that the dead time refers to a short-circuit prevention period setto prevent a short-circuit current from flowing between an upper-armIGBT device and a lower-arm IGBT device (e.g., between IGBT device Tr1and IGBT device Tr7) in the DC/DC converter 60.

Next, the reduced-performance area avoidance control for the DC/DCconverter 60 which is executed by the controller 80 at predeterminedmoments (e.g., when the operation is started or stopped, or at constanttime intervals during the operation) will be described in detail withreference to FIG. 3. Note that the description below assumes the casewhere a three-phase drive is set as the initial setting for the DC/DCconverter 60.

FIG. 3 is a flowchart showing the reduced performance area avoidancecontrol processing according to the first embodiment.

The controller 80 obtains the power required for the entire system basedon signals (acceleration-opening-degree signal, etc.) output from thevarious sensors, and determines the power distribution between the fuelcell stack 20 and the secondary battery 50 (step S301).

The controller (calculation unit) 80 judges whether or not the passingpower of the DC/DC converter 60 which is obtained in accordance with thepower distribution falls within the reduced response performance areafor the number of active phases as of the present moment (here, threephases) (step S302).

If the controller 80 determines that the passing power of the DC/DCconverter 60 does not fall within the reduced response performance area(step S302: NO), this indicates that the DC/DC converter 60 is beingdriven suitably (i.e., the voltage controllability has not beendegraded). Thus, the controller 80 leaves the processing of this routineto end the processing.

Meanwhile, if the controller (number-of-phases control unit) 80determines that the passing power of the DC/DC converter 60 falls withinthe reduced response performance area (step S302: YES), the controller80 determines the number of phases which avoids the drive within thereduced response performance area (i.e., the changed number of activephases) (step S303). The controller (setting unit, drive control unit)80 outputs (sets) a command for switching to the determined number ofphases (phase switching command) to the DC/DC converter 60 (step S304),drives the DC/DC converter 60 with the number of phases after theswitching and then ends the processing.

As described above, when the passing power of the DC/DC converter 60falls within the reduced response performance area for the number ofactive phases as of the relevant moment, the number of active phases forthe DC/DC converter 60 is switched. This enables the drive of the DC/DCconverter 60 which avoids the reduced response performance area, therebyimproving the voltage controllability of the DC/DC converter 60 comparedto related art.

Here, the example above shows switching between a three phase and asingle phase as switching of the number of active phases for the DC/DCconverter 60, but switching may be performed between three phases, twophases or a single phase. The number of phases for switching may bearbitrarily set in accordance with the number of active phases N (N≧2)for the mounted DC/DC converter 60. Further, when it is determined instep S302 that the reduced response performance area cannot be avoidedin any number of active phases, the controller 80 may select the mostenergy-efficient number of active phases (e.g. three phases) from amongthe possible number of active phases for switching. As described above,the technical idea in which the number of active phases for the DC/DCconverter 60 is determined in consideration of energy efficiency may beapplied to not only the case where it is determined that the reducedresponse performance area cannot be avoided but also to the case whereit is determined that the passing power does not fall within the reducedresponse performance area.

B. Second Embodiment

FIG. 4 is a graph showing the relationship between a passing power and adead time compensation value of the DC/DC converter 60 for a certainnumber of active phases. In the first embodiment above, the drive of theDC/DC converter 60 which avoids the reduced response performance area isenabled by changing the number of active phases. In the secondembodiment, the drive of the DC/DC converter 60 which avoids the reducedresponse performance area is enabled by changing the passing power.

As shown in FIG. 4, a negative reduced response performance area and apositive reduced response performance area exist in a three-phase drive.Here, when the passing power of the DC/DC converter 60 obtained inaccordance with the power distribution falls within, e.g., the positivereduced response performance area (see passing power a in FIG. 4), thepassing power of the DC/DC converter 60 is shifted in a direction inwhich an amount of assist by the secondary battery 50 increases(positive direction) without any influence on a system output, therebyavoiding the reduced response performance area (see passing powera→passing power a′ in FIG. 4).

On the other hand, when the passing power of the DC/DC converter 60obtained in accordance with the power distribution falls within, e.g.,the negative reduced response performance area (see passing power b inFIG. 4), the passing power of the DC/DC converter 60 is shifted in adirection in which an amount of power generated by the fuel cell stack20 increases (negative direction) without any influence on the systemoutput, thereby avoiding the reduced response performance area (seepassing power b→passing power b′ in FIG. 4): Note that the surplus powergenerated by shifting the passing power of the DC/DC converter 60 to thepositive side may be stored in the secondary battery 50, consumed by thevehicle auxiliary apparatus 70, or converted to thermal energy to bereleased to the atmosphere. Further, when the passing power of the DC/DCconverter 60 is shifted to the negative side, it is sufficient thatpower may be supplied from the secondary battery 50 to make up for adeficiency of power. As described above, the reduced responseperformance area may be avoided by changing the passing power.

Next, the reduced-performance area avoidance control for the DC/DCconverter 60 which is executed by the controller 80 at predeterminedmoments (e.g., when the operation is started or stopped, or at constanttime intervals during the operation) will be described in detail withreference to FIG. 5.

FIG. 5 is a flowchart showing the reduced performance area avoidancecontrol processing according to the second embodiment.

The controller 80 obtains the power required for the entire system basedon signals (acceleration-opening-degree signal, etc.) output from thevarious sensors, and determines the power distribution between the fuelcell stack 20 and the secondary battery 50 (step S401).

The controller (calculation unit) 80 judges whether or not the passingpower of the DC/DC converter 60 which is obtained in accordance with thepower distribution falls within either the positive or negative reducedresponse performance area (step S402).

If the controller 80 determines that the passing power of the DC/DCconverter 60 does not fall within the reduced response performance area(step S402: NO), this indicates that the DC/DC converter 60 is beingdriven suitably (i.e., the voltage controllability has not beendegraded). Thus, the controller 80 leaves the processing of this routineto end the processing.

Meanwhile, if the controller (power control unit) 80 determines that thepassing power of the DC/DC converter 60 falls within the reducedresponse performance area (step S402: YES), the controller 80 determinesa passing power after the shift so as to shift the passing power of theDC/DC converter 60 without causing a significant influence on the systemoutput (step S403). For example, when the passing power of the DC/DCconverter 60 falls within, e.g., the positive reduced responseperformance area, the passing power of the DC/DC converter 60 is shiftedin a direction in which an amount of assist by the secondary battery 50increases (positive direction) without any influence on the systemoutput, thereby avoiding the reduced response performance area (seepassing power a→passing power a′ in FIG. 4).

On the other hand, when the passing power of the DC/DC converter 60obtained in accordance with the power distribution falls within, e.g.,the negative reduced response performance area, the passing power of theDC/DC converter 60 is shifted in a direction in which an amount of powergenerated by the fuel cell stack 20 increases (negative direction)without any influence on the system output, thereby avoiding the reducedresponse performance area (see passing power b→passing power b′ in FIG.4). Specifically, the controller (drive control unit) 80 outputs a powershift command to the DC/DC converter 60 in order to obtain thedetermined passing power after the shift (step S404), controls the driveof the DC/DC converter 60 so as to obtain the passing power after theshift, and then ends the processing.

As described above, the drive of the DC/DC converter which avoids thereduced response performance area may also be enabled by shifting thepassing power of the DC/DC converter.

C. Third Embodiment

FIG. 6 is a graph showing the relationship between a passing power and adead time compensation value of the DC/DC converter 60 for a certainnumber of active phases. The solid line indicates the graph for the casewhere a switching control of the DC/DC converter 60 is carried out witha control signal with a carrier frequency Fn, and the dotted lineindicates the graph for the case where a switching control of the DC/DCconverter 60 is carried out with a control signal with a carrierfrequency Fm (<Fn). In the second embodiment above, the drive of theDC/DC converter 60 which avoids the reduced response performance area isenabled by changing the passing power. In a third embodiment, the driveof the DC/DC converter 60 which avoids the reduced response performancearea is enabled by changing the carrier frequency.

As shown in FIG. 6, a reduced response performance area in which a deadtime compensation value varies greatly changes in accordance with thecarrier frequency of the DC/DC converter 60. This is because a variationΔI of current flowing in the reactors of the DC/DC converter 60 variesby changing a carrier frequency F, and the dead time compensation valuevaries accordingly.ΔI=V*ΔT/LΔT=1/F

V represents a voltage, L represents an inductance, I represents acurrent, and F represents a carrier frequency.

FIG. 7 shows the relationship between a control signal for carrying outa switching control over IGBT devices Tr1 to Tr12 and the ripple currentflowing in reactors L1 to L3. For the convenience of explanation, thecase of a single-phase operation will be described as an example. A timeperiod Tn indicates a time period during which IGBT devices Tr1 and Tr10are turned on, and a time period Tp indicates a time period during whichIGBT devices Tr4 and Tr7 are turned on. A carrier cycle corresponds toTn+Tp. Assuming that the maximum value of the ripple current isrepresented by In and that the minimum value thereof is represented byIp, the ripple current width corresponds to In−Ip. ZP represents a pointof zero crossing of the ripple current (hereinafter referred to as azero crossing point).

When the zero crossing point ZP exists, the direction (sign) of theripple current is often reversed, and thus the voltage controlperformance of the DC/DC converter 60 is reduced remarkably. Therefore,the zero crossing point ZP emerges in an area where a dead timecompensation value varies greatly, i.e., a reduced response performancearea. Meanwhile, when the maximum value In is a negative value or whenthe minimum value Ip is a positive value, the zero crossing point ZPdoes not exist, and thus the DC/DC converter 60 has satisfactory voltagecontrol performance. Further, when the zero crossing point ZP exists atthe center of the ripple current width, the sign of the ripple currentis reversed symmetrically with respect to the zero crossing point ZP,and thus the DC/DC converter 60 has satisfactory voltage controlperformance.

As shown in FIG. 7, as the carrier frequency is increased, the timeperiods Tn and TP become shorter. Thus, it can be understood that theripple current width becomes smaller. Conversely, as the carrierfrequency is decreased, the time periods Tn and TP become longer, andthus the ripple current width becomes larger. When the ripple currentwidth is changed, the point of zero crossing of the ripple currentvaries accordingly. Therefore, the carrier frequency is changed, therebymoving the operating point of the DC/DC converter 60 out of the reducedresponse performance area.

Next, the reduced-performance area avoidance control of the DC/DCconverter 60 which is executed by the controller 80 at predeterminedmoments (e.g., when the operation is started or stopped, or at constanttime intervals during the operation) will be described in detail withreference to FIG. 8.

FIG. 8 is a flowchart showing the reduced performance area avoidancecontrol processing according to the third embodiment.

The controller 80 obtains the power required for the entire system basedon signals (acceleration-opening-degree signal, etc.) output from thevarious sensors, and determines the power distribution between the fuelcell stack 20 and the secondary battery 50 (step S501).

The controller (calculation unit) 80 judges whether or not the passingpower of the DC/DC converter 60 which is obtained in accordance with thepower distribution falls within the reduced response performance area(step S502).

If the controller 80 determines that the passing power of the DC/DCconverter 60 does not fall within the reduced response performance area(step S502: NO), this indicates that the DC/DC converter 60 is beingdriven suitably (i.e., the voltage controllability has not beendegraded). Thus, the controller 80 leaves the processing of this routineto end the processing.

Meanwhile, if the controller (frequency control unit) 80 determines thatthe passing power of the DC/DC converter 60 falls within the reducedresponse performance area (step S502: YES), the controller 80 determinesa carrier frequency after a change in order to avoid the reducedresponse performance area (step S503). The controller (drive controlunit) 80 then outputs a carrier-frequency change command (e.g., thecarrier frequency Fm→Fn) to the DC/DC converter 60 (step S504), controlsthe drive of the DC/DC converter 60 with the carrier frequency after thechange and then ends the processing.

As described above, the drive of the DC/DC converter which avoids thereduced response performance area may also be enabled by changing thecarrier frequency.

D. Modification

The configurations of the embodiments described above may be combinedarbitrarily to control the various parameters (the number of activephases, passing power and carrier frequency) of the DC/DC converter 60to be in the optimum state in accordance with the power variation,thereby improving the voltage controllability of the DC/DC converter 60.

The control of the DC/DC converter 60 which is executed by thecontroller 80 at predetermined moments (e.g., when the operation isstarted or stopped, or at constant time intervals during the operation)will hereinafter be described in detail with reference to FIG. 9.

FIG. 9 is a flowchart showing control processing for the DC/DC converter60 according to a modification.

The controller calculates a rate of change of an acceleration openingdegree based on, e.g., an acceleration-opening-degree signal, which isdetected in succession by the acceleration sensor (sensor) 93 (stepS701). The controller (detection unit, judgment unit) 80 then proceedsto step S702, and compares the obtained rate of change of accelerationopening degree with a preset sudden-change determination threshold(setting threshold) and judges whether or not the obtained rate ofchange of acceleration opening degree exceeds the sudden-changedetermination threshold (i.e., whether or not a request voltage for thefuel cell stack 20 has suddenly changed) (step S702). Here, thesudden-change determination threshold is obtained in advance throughexperiments, etc., and refers to the rate of change of accelerationopening degree for the case of a sudden change of the request voltagefor the fuel cell stack 20.

If the controller 80 determines that the request voltage for the fuelcell stack 20 has not changed suddenly (step S702: NO), the controller80 leaves the processing of this routine to end the processing.Meanwhile, if the controller (parameter control unit) 80 determines thatthe request voltage for the fuel cell stack 20 has changed suddenly(step S702: YES), the controller 80 controls the parameters, i.e., thenumber of active phases, passing power and carrier frequency to be inthe optimum state in order to improve the voltage controllability of theDC/DC converter 60 (step S703). This will be described with an example.The number of active phases for the DC/DC converter 60 is switched toobtain the minimum current ripple. The carrier frequency of the DC/DCconverter 60 is changed to a controllable frequency that is the closestto a duty update cycle. Further, regarding the passing power of theDC/DC converter 60, the passing power is shifted so as to avoid thereduced response performance area based on the determined number ofactive phases (e.g., three phases) and carrier frequency (e.g., carrierfrequency Fm). The controller 80 controls the drive of the DC/DCconverter as described above, and then ends the processing.

As described above, according to the embodiment above, the variousparameters of the DC/DC converter 60 are switched to be brought into theoptimum state even when the request voltage for the fuel cell stack 20suddenly changes due to a large change in acceleration opening degree(at the time of rapid start, during a rapid acceleration, etc.), therebyimproving the voltage controllability of the DC/DC converter 60.

Note that, regarding the control of the DC/DC converter 60 for the caseof a sudden change of the request voltage for the fuel cell stack 20,all of the three parameters (the number of active phases, passing powerand carrier frequency) may be treated as objects of switching, and also,any one parameter (e.g., the number of active phases) or two parameters(e.g., passing power and carrier frequency) of the parameters may betreated as an object(s) of switching. Further, whether or not therequest voltage for the fuel cell stack 20 has changed suddenly may bejudged based on various signals such as the vehicle speed signaldetected by the vehicle speed sensor 92 and the request power signalfrom the FC auxiliary apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a primary configuration of a fuel cell systemaccording to a first embodiment.

FIG. 2 is a graph showing the relationship between a passing power and adead time compensation value of a DC/DC converter according to the firstembodiment.

FIG. 3 is a flowchart showing reduced performance area avoidance controlprocessing according to the first embodiment.

FIG. 4 is a graph showing the relationship between a passing power and adead time compensation value of a DC/DC converter according to a secondembodiment.

FIG. 5 is a flowchart showing reduced performance area avoidance controlprocessing according to the second embodiment.

FIG. 6 is a graph showing the relationship between a passing power and adead time compensation value of a DC/DC converter according to a thirdembodiment.

FIG. 7 is a diagram showing the relationship between a control signaland a ripple current in the DC/DC converter according to the thirdembodiment.

FIG. 8 is a flowchart showing reduced performance area avoidance controlprocessing according to the third embodiment.

FIG. 9 is a flowchart showing control processing for a DC/DC converteraccording to a modification.

DESCRIPTION OF REFERENCE NUMERALS

10: fuel cell system, 20: fuel cell stack, 21: FC auxiliary apparatus,22: cell voltage detector, 30: traction inverter, 40: traction motor,50: secondary battery, 60: DC/DC converter, 70: vehicle auxiliaryapparatus, 80: controller

What is claimed is:
 1. A fuel cell system comprising: a DC/DC converterfor increasing/decreasing an output voltage of a fuel cell; acalculation unit which is programmed to calculate a passing power of theDC/DC converter; a power control unit which is programmed to determine,when the calculated passing power falls within a reduced responseperformance area, the passing power of the DC/DC converter so that thepassing power does not fall within the reduced response performancearea; and a drive control unit that drives the DC/DC converter so as toobtain the determined passing power.